The present invention relates to an improvement in a thermal fixing apparatus employed in a device such as a copying machine, electronic printer or the like which is used to fix a toner image on a support such as a paper sheet passed between a pair of mutually press-contacted rollers, at least one of which performs a heating function.
Some thermal roll-type fixing apparatuses used to perform the fixing function in a copying machine or electronic printer have previously been known (see, for instance, Laid Open Japanese Patent Applications Nos. 49-81041 and 53-145635 and Japanese Patent Publication No. 54-19347). One example of these apparatuses comprises a pair of rollers, one of which is composed of a heat-resist elastic layer of a material such as silicone rubber or the like and/or a refractory material of layer such as polytetrafluoroethylene coated on the outer surface of a metallic hollow cylinder which is provided with a heat source such as an infrared lamp, halogen lamp or nichrome wire therein, and the another roll of a metallic hollow cylinder without a heating source. A sectional drawing of a conventional apparatus of this type is depicted in FIG. 1. In FIG. 1, reference numerals 1, 2 and 3 indicate, respectively, a heated roll, a pressure roll, and side plates for supporting the heat roll and the pressure roll at the two ends thereof through bearings (not shown). Reference numerals 4 and 5 respectively designate a paper feeding guide and a paper carrying thereon a toner image 6.
The heat roll 1 is formed by covering the outer surface of a hollow metal cylinder 8 with a refractory parting agent layer 9. A heat source 7 is positioned in the interior of the metal cylinder 8. The pressure roll 2 is constructed by covering the outer surface of a hollow metal cylinder 10 with an elastic and refractory material of layer 11. The rolls 1 and 2 are mounted in mutual press-contact with one another so as to form a nip through which the paper carrying the toner image passes. The pressure applied between the two rolls is controlled to provide a predetermined nip width in the direction in which the paper is fed (referred to as a contact width, later in this specification).
In operation, the heat source 7 is activated to supply heat to the surface of the heat roller 1 through the metal core and insulating layer. The rolls 1 and 2 are rotated in the directions indicated by arrows in FIG. 1 to convey the paper 5 along the paper feeding guide 4 in the direction indicated by another arrow.
The type of fixing apparatus shown in FIG. 1 is advantageous when compared with other thermal fixing apparatuses such as the oven type and flash lamp type in that the energy consumption is less, the mechanism is more reliable, and a higher speed fixing operation is provided. However, this apparatus is disadvantageous in that a relatively long preheating time such as at least one minute is required for the temperature of the surface of the heat roll to be raised to an operating temperature range on the order of 150.degree. to 200.degree. C. after the power source is turned on.
To reduce this preheating time, the diameter or the thickness of the hollow cylinder of the roll may be decreased to thereby decrease the heat capacity of the roll. Doing so, however, results in the requirement that greater pressure must be exerted to press-contact the rollers. Although ordinarily 0.5 to 1.0 kg/cm of pressure is exerted, so that sufficient contact width is formed in order to ensure sufficient contact time for fixing. Also, the reduction in the diameter or thickness of the roll lessens the rigidity of the roll to resist the greater pressure to be applied. In the case of insufficient pressure for press-contact of rollers, there occurs insufficient pressure for fixing and an insufficient contact width in the longitudinal center of rollers which cause a poor fixing. Still further, providing more power to the heater suffers from a drawback in that during preheating the heat source may cause the temperature at the surface of the roll to overshoot the desired operating temperature, thereby resulting in a longer stabilization time and no reduction of the preheating time.
Another approach, such as disclosed in Laid Open Japanese Patent Publications Nos. 50-27550, 50-93658, 50-161244, 51-9457 and 54-83440, is to provide an auxiliary heating source to shorten the preheating time
However, this prior approach has not been put to practical use, because the paper tends to be burned in the case of contact with the auxiliary heating source and the thermal efficiency is low.